1. Field of Invention
This invention relates to a method of hardening the surface of metals or alloys comprised of, or containing, elements which form hard carbides, especially titanium and its alloys.
2. Prior Art
Much research and effort was expended to devise methods of winning titanium metal from its ores, and purifying it from environmental contaminants, especially oxygen, nitrogen and carbon, the presence of which had previously rendered the metal non-ductile.
When the ductile metal became available, alloy development was stimulated. Unlike carbon steel alloys, titanium-carbon alloys have no soft phase in which they can be machined and formed with relative ease. If enough carbon is added to provide hardness equivalent to steel, the alloy becomes unmachineable and loses both cold and hot ductility regardless of phase disposition.
When titanium-aluminum alloys were developed, strengths greater than the titanium-carbon alloys were achieved while still preserving workability. Of the presently available titanium grades and alloys, none has a carbon content over 0.1%, nor a hardness over RC 42.
In absence of a method of hardening the surface of such alloys, they are of limited applicability for certain uses, despite their very favorable yield strength to weight ratio, corrosion resistance and many other virtues. Machine parts in rubbing or sliding contact, or which are exposed to abrasive environmental conditions, should have a surface hardness equivalent to RC 55 or higher, to prevent galling, seizing and wear.
Of the titanium compounds, both the carbide and the nitride are among the hardest materials known. Diverse methods of manufacture are known, but the carbide can be formed by reacting the oxide or the metal with carbon at high temperatures, in a vacuum or under argon. The nitride can be formed at much lower temperatures by very slow absorption of nitrogen from the pure gas or from ammonia, in a pressurized vessel.
Although initially it was hoped that titanium carbide would become a new industrial abrasive because of its excellent hardness and observed oxidation resistance, it was soon discovered that the material became rapidly blunted in use. Oxidation and other reactions with the workpiece were suspected to cause wear of the abrasive grains.
So far as I am aware, no proposal has been made to overcome this problem, to reduce or renew the cutting surface, such as described in the present disclosure.
Attempting to combine the virtues of each material, Alexander, U.S. Pat. No. 2,674,542 (1951), proposed a method of vacuum brazing carbide particles to the surface of titanium, allowing the carrier alloy to diffuse into the surface of the metal. Subsequent research revealed the presence and liability of amorphous intermetallic compounds which would be formed by the carrier.
Nitriding of titanium articles was developed by 1953. But due to extreme surface stresses induced by the nitride film, extensive precautions must be taken to avoid dimensional changes during the nitriding, and during and subsequent to finish machining. Parts containing thin sections or acute edges cannot be easily nitrided because extreme brittleness results at these locations.
A sintered titanium-titanium carbide alloy was disclosed by Frehn, U.S. Pat. No. 3,737,290 (1973). But the size, shape, and production quantity limitations intrinsic to powder metallurgy are compounded, in this invention, by the short life of tooling exposed to severe contact with the hard particles.
More recent techniques for surface hardening titanium include ion implantation and ion plating, each producing extremely thin films, and laser melt/particle injection. In the latter process, a moving laser beam liquifies the surface while a stream of hard particles is fed into the molten pool. These processes are a conducted in vacuum.
The laser melt/particle injection process, especially, implies that it has not heretofore been sufficiently appreciated how rapidly carbon combines with molten titanium, the extent of titanium's preference for carbon over oxygen at high temperatures, or the extreme instantaneous temperatures produceable by electric arcs. In short, that carbide particles are produceable in situ on the surface of titanium by combination with carbon, in the natural atmosphere, rather than by the degrading of expensive particles by expensive equipment, in vacuum.
In the field of ferrous metals, two discoveries are of interest. In carbon arc welding and cutting, it had long been observed that the carbon arc offered a certain degree of protection against oxidation of the workpiece, but that the workpiece became excessively carburized, especially if the supply current was the wrong polarity. Opinion varied as to which was the correct polarity to minimize this phenomenon, but the result was not in doubt: the metal so effected became unmachineable.
In hardfacing, it was discovered that a micro-spark welding process could be used to weld a carbide facing to a steel tool surface. A hand held vibrator supported the carbide electrode while a variable transformer supplied a small welding current.
In summary, then, of the situation regarding titanium: No alloy is available which has adequate surface hardness for many applications. Presently known methods of hardening the surface of titanium and its alloys are elaborate, lengthy, unwieldy, require expensive furnaces or other heat sources, and a vacuum or controlled atmosphere environment, are unsuited for many applications or have other undesireable limitations.
Most users of titanium and titanium alloys, therefore, would welcome a method of hardening the metal surface that is convenient, economical, efficient and effective, does not require furnaces, lasers, vacuum pumps, or other expensive and bulky equipment, and whereby a finished surface can be produced upon a single article in a relatively short time.
Most users of ferrous alloys, or other alloys containing carbide forming constituents would welcome a method of surface hardening more adhesive than plating, more compatible and less distorting than welding, more accurately localized than case hardening, nitriding, induction hardening, and more economical by far in many instances.